Break it into two right triangles one with b = 3 h = 6
the other with b = 12 h = 6
area of right triangle = 1/2 b * h so total would be 1/2 3 * 6 + 1/2 6 *12 = 45 units2
If you need the leg lengths of the entire right triangle ...use the distance formula between two points
L leg = sqrt ( (3-0)^2 + (0-6)^2 ) = sqrt 45
R leg = sqrt ( 12^2 + 6^2) = sqrt 180
area = 1/2 L1 * L2 = 1/2 (sqrt (45) * sqrt180 ) = 45 units2 (again)